U.S. Pat. No. 4,937,447 to Barrett discloses and claims apparatus for detecting air turbulence (windshear as well as clear air turbulence) in navigable airspace by remotely sensing changes in the spatial temperature profile in the air column ahead of an aircraft by repeatedly sensing the intensity of the thermal radiation from the column of atmospheric air in at least two spectral peaks in the 4.17 .mu.m to 4.2 .mu.m region of the CO.sub.2 spectral emission band, and comparing the relative intensity of the spectral peaks over time. The apparatus utilizes a tunable Fabry-Perot etalon in combination with a bandpass filter for selecting the spectral emission bands. Tunable Fabry-Perot etalons are relatively expensive, delicate components. The present invention provides simplified apparatus which avoids the need for the Fabry-Perot etalon, by substituting a rotatably mounted bandpass filter for the etalon/filter combination of the prior art. The filter has a broader transmission region than the Fabry-Perot etalon. Accordingly, in the present embodiment, the relative intensity of such broader spectral regions (which may contain one or more spectral peaks) within the 4.17 .mu.m to 4.2 .mu.m region of the CO.sub.2 spectral emission band is compared.